An important component of disposable absorbent articles such as diapers is an absorbent core structure comprising water-absorbing polymers, typically hydrogel-forming water-absorbing polymers, also referred to as absorbent gelling material, AGM, or super-absorbent polymers, SAPs. This polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness.
Especially useful water-absorbing polymers or SAPs are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or polyfunctional monomers such as N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or polyfunctional monomer materials serve to lightly crosslink the polymer chains thereby rendering them water-insoluble, yet water-absorbing. These lightly crosslinked absorbent polymers contain a multiplicity of carboxylate groups attached to the polymer backbone. It is generally believed that the neutralized carboxylate groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network. In addition, the polymer particles are often treated as to form a surface cross-linked layer on the outer surface in order to improve their properties in particular for application in baby diapers.
Water-absorbing (hydrogel-forming) polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequately high absorption capacity, as well as adequately high gel strength. Absorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen polymer particles to resist deformation under an applied stress. The gel strength needs to be high enough in the absorbent member or article so that the particles do not deform and fill the capillary void spaces to an unacceptable degree causing so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake or the fluid distribution, i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article and leakage from the absorbent article can take place well before the water-absorbing polymer particles are fully saturated or before the fluid can diffuse or wick past the “gel blocking” particles into the rest of the absorbent article. Thus, it is important that the water-absorbing polymers (when incorporated in an absorbent structure or article) maintain a high wet-porosity and have a high resistance against deformation thus yielding high permeability for fluid transport through the swollen gel bed.
Surface crosslinking leads to a higher crosslinking density close to the surface of each water-absorbent particle. This addresses the problem of “gel blocking”, which means that, with earlier types of water-absorbents, a liquid insult will cause swelling of the outermost layer of particles of a bulk of water-absorbent particles into a practically continuous gel layer, which effectively blocks transport of further amounts of liquid (such as a second insult) to unused water-absorbent below the gel layer. While this is a desired effect in some applications of water-absorbents (for example sealing underwater cables), it leads to undesirable effects when occurring in personal hygiene products. Increasing the stiffness of individual gel particles by surface crosslinking leads to open channels between the individual gel particles within the gel layer and thus facilitates liquids transport through the gel layer. Although surface crosslinking decreases the CRC or other parameters describing the total absorption capacity of a water-absorbent sample, it may well increase the total amount of liquid that can be absorbed by a hygiene product containing a given amount of water-absorbent during normal use of the product.
There is still a need to provide thinner absorbent articles since they increase the wearing comfort. There has been a trend to remove part or all of the cellulose fibres (pulp) from the products. These ultrathin hygiene articles may comprise construction elements (for example—but not limited to—the diaper core or the acquisition distribution layer) which consist of water-absorbing polymeric particles to an extent which is in the range from 50% to 100% by weight, so that the polymeric particles in use not only perform the storage function for the fluid but also ensure active fluid transportation (in simple words, the capacity of a swollen gel bed to pull liquid against gravity, or wicking absorption, a property that can be quantified as Fixed Height Absorption (“FHA”) value, determined as described below) and passive fluid transportation (in simple words, the capacity of a swollen gel bed to allow flow of liquid with gravity, a property that can be quantified as Saline Flow Conductivity (“SFC”) value, determined as described below). The greater the proportion of cellulose pulp which is replaced by water-absorbing polymeric particles or synthetic fibers, the greater the number of transportation functions which the water-absorbing polymeric particles have to perform in addition to their storage function. It has been found that for such absorbent articles in particular, there is a need for water-absorbent polymeric particles that have a good absorbent capacity (CRC value) and a good fluid transportation (reflected by a good FHA value and SFC value). Furthermore, it is required that the water-absorbing polymeric particles have a sufficiently high initial uptake rate that can be quantified as Free Swell Rate (FSR). It is well-known in the art that there is a trade-off between the wicking ability and the initial uptake rate.
WO 2009/016055 discloses water-absorbing polymeric particles with high fluid transportation and absorption performance by contacting polymer particles with a postcrosslinker, a nitrogen-containing water-soluble polymer and a hydrophobic polymer and heat-treating the obtained particles.
WO 2006/082239 discloses a water-absorbing material having a coating of elastic film-forming polymers which have high core shell centrifuge retention capacity (CS-CRC), high core shell absorbency under load (CS-AUL) and high core shell saline flow conductivity (CS-SFC).
It is an object of the invention to provide a water-absorbing material having a high active fluid transportation (FHA) and a high initial uptake rate (FSR).
It is a further object of the invention to provide a water-absorbing material having in addition a high core shell saline flow conductivity (CS-SFC).